In the prior art, bell-forming machines usually comprise a workhead provided with at least one kiln for heating the pipes and a forming device, both associated to a bench provided with suitable means for moving the pipes, on which bench the pipes are transported in a continuous cycle.
The pipe, during the forming process, is held in the work position by vises having jaws which are mobile in a vertical plane.
The prior art in this filed teaches substantially two types of vise. In a first type the jaws are slidable along guide columns, while in a second type, realized by the present applicant, the guide columns are absent. In the latter case, the jaws are mobile independently one from the other and are reciprocally aligned thanks to the structure of the machine which supports them.
In all bell-forming machines, the various pipe sections, consisting of straight lengths of pipe, are sent one by one and continuously on to the bench, along a direction coinciding with the axis of symmetry of the pipe.
After having reached the bench, the pipe sections are turned transversally to their original direction, through intermittent step-movements, remaining all the time parallel to one another. During the pauses between one step and a next, the pipes undergo the single work operations which essentially consist in heating up (in the kiln) the end thereof to be belled, followed by constraining in the vise and, finally, the forming operation. Both the kiln for the heating phase and the forming device for the bell-forming phase are mobile on trucks, and are neared and distanced from the pipe during the various work phases.
A first drawback of the bell-forming machines provided with jaws on guide columns is that during the work cycle the pipe being formed has to be moved two extra times in a perpendicular direction to the vertical plane on which the jaws move, in order not to interfere with the guide columns. A first of the two movements transports the shaped ends past the guide columns before the ends are gripped in the vise, so that the end of the pipe to be formed can be neared to the forming device located (with respect to the vise) on the opposite side to the pipe end on the bench. Then, when the forming operation has been completed, a movement in a contrary direction to the first has to be performed to bring the pipe back into its original position on the bench.
As can be gleaned from the above, the work cycle of bell-forming machines having guide columns is longer and less economical than that of similar machines not having guide columns.
The type not provided with guide columns also present drawbacks, connected mostly with the fact that they are mechanically and electronically more complex, and therefore more expensive.
Their greater complexity is due to the fact that bell-forming machines having pressurised-fluid forming chambers operating with an elastic wall must be further equipped with rigid striker elements to meet and support the elastic wall during a pressurisation phase.
These striker elements, in bell-forming machines provided with guide columns, are easily positionable on the jaws themselves. In bell-forming machines not provided with guide columns, a supplementary truck has to be provided for supporting said elements, and naturally said truck must, in its movements, be synchronized with the remaining parts of the machine.